Sonic transducers are widely used for a number of applications including medical imaging, cleaning systems or scrubbers used in fabricating semiconductor or Micro-Electromechanical System (MEMS) devices. In a typical cleaning system substrates, such as silicon wafers, are immersed in a liquid to which sonic energy is applied. High intensity sound waves generate pressure fluctuations that lead to cavitation, a condition in which millions of microscopic bubbles rapidly form and collapse in the liquid. The collapse of these cavitation bubbles produce shock waves that impinge on substrate surfaces, dislodging particles thereon. Conventional cleaning systems use typically piezoelectric transducers operating at ultrasonic frequencies of less than about 400 kHz to apply sonic energy to the liquid. However, as the sizes of elements or features in semiconductor circuit MEMS devices continues to shrink, the trend in sonic cleaning systems has been toward transducers capable of operating at higher frequencies, which produce smaller cavitation bubbles that increase the cleaning effectiveness, and provide a more gentle cleaning while reducing probability of damage to the substrate. Unfortunately, the operating frequency or resonant frequency of piezoelectric transducers is determined by a film thickness of the piezoelectric material and is generally limited to the ultrasonic or low megasonic frequency range.
Accordingly, there is a need for a transducer suitable for use in cleaning systems and capable of operating over the full megasonic range.